1. Field of Invention
The invention generally relates to a method and system for improving an optical communications network. More particularly, the invention relates to a method and system that suppresses ring lasing or ASE (amplified stimulated emission) peaking in a wavelength division multiplexed (WDM) optical communications network.
2. Description of Related Art
Wavelength Division Multiplexing (WDM) is used to transmit a plurality of optical channels via an optical wave guide medium where each channel carries information signals within a network or system. Each channel within the WDM signal is associated with a particular wavelength (actually, each channel includes a range of wavelengths but is commonly referred to using its center wavelength), thereby increasing the information capacity of fiber optic systems. Such optical systems include, for example, local area networks (LANs), telecommunication systems and cable television systems. Depending on the optical system configuration and the transmission path, various optical and/or electrical amplifiers are required to provide the necessary optical power to efficiently transmit and amplify the WDM signal to each component within the network.
WDM optical networks have traditionally been used for long haul point to point (linear) networks. However, with the increasing demands on communication systems, WDM optical networks can also be used in smaller system configurations, such as local or regional telephone or data networks. In these systems, communication signals are usually transmitted over a limited geographic area to various nodes within a network. A particular node can be configured to drop one or more information bearing or payload channels from the WDM signal, process the information contained in the dropped channels and add the channels containing new information to the WDM signal for transmission to other nodes in the network.
An optical add/drop multiplexer (OADM) is typically be used at a node to drop one or more channels from the WDM signal while permitting the remaining channels (express channels) to continue on the optical transmission system. Channels may be added to the WDM signal downstream of the dropped (blocked) channels.
To drop a channel, an optical blocking filter is used to filter out the channel(s) being dropped from the WDM signal. Various forms and types of such blocking filters are used such as a fiber Bragg gratings (FBG), thin film filters, etc. Such blocking filters are typically transmissive with respect to the express channels (those channels passing through the OADM) and reflective with respect to the dropped channels. U.S. Pat. No. 5,748,349 illustrates one example of a known OADM that utilizes a series of Bragg gratings to drop channels.
In metropolitan or regional areas, WDM systems having a ring configuration can be used to provide high capacity data links between several nodes. In other words, WDM networks may be configured in a ring topology in which the channel wavelengths may circulate around the ring. Such ring topologies are typically utilized in metropolitan areas. WDM rings carry multiple optical signals (channels) on a looped optical communication path.
It is also customary for wavelengths to be dropped (“blocked”) from the ring by placing an optical filter on the ring. Such a blocking filter presents a large loss in the transmission path over its wavelength interval of interest (stop band), over and above the ring's fiber loss. Each unique wavelength placed on a ring will have an associated filter to drop it from the ring.
Unused wavelengths, however, may not have associated blocking filters deployed on the ring and so the ring's transmission loss at these wavelengths is the same as the fiber loss.
Fiber optic ring networks may employing optical amplifiers (OAs) to overcome the transmission losses associated with the fiber and optical elements situated on the ring. While OAs compensate for transmission loss by amplifying the signals present on the ring, they also generate noise (ASE) over the entire spectrum of the OAs bandwidth. ASE power lying within the blocking filter's stop band is attenuated. If portions of the optical spectrum are not blocked, then ASE at those unblocked wavelengths will re-circulate around the ring. If the round-trip gain of the ring exceeds unity, then the ASE power at unblocked wavelengths will increase due to positive feedback. The ASE power that grows under such conditions may actually cause lasing, but more commonly, it simply creates power fluctuations for the signals carried on the ring. These power instabilities may disrupt the useful signals' quality of transmission and are undesirable.